从java反编译及字节码角度探索分析String拼接字符串效率

目录

• 前言
• 拼接场景
• 示例 1：
• 示例 2：
• 字节码信息
• 总结

前言

又刷到这篇文章

为什么idea建议去掉StringBuilder，使用“+”拼接字符串

网上有很多文章用 JUnit 进行测试验证，可以网上搜一下，这里不做赘述。今天我们从反编译和字节码的角度分析字符串拼接的时候底层到底做了什么

拼接场景

示例 1：

public class Hello2 {
    public static void main(String[] args) {
    }
    public void add1() {
        String a = "aaa" + "bbb" + "ccc";
        String b = new StringBuilder("aaa").append("bbb").append("ccc").toString();
    }
}

反编译结果：

package com.noah.nowcoder;
public class Hello2 {
  public static void main(String[] args) {}
  public void add1() {
    String a = "aaabbbccc";
    String b = "aaa" + "bbb" + "ccc";
  }
}

Java 编译器优化（JDK 版本相关），编译结果可以直接看出， String a = "aaa" + "bbb" + "ccc" 执行效率更高；

示例 2：

public class Hello {
    public static void main(String[] args) {
        String str1 = "";
        for (int i = 0; i < 100000; i++) {
            str1 += "-" + UUID.randomUUID().toString();
        }
        System.out.println(str1);
        StringBuilder stringBuilder = new StringBuilder();
        for(int i = 0; i < 100000; ++i) {
            stringBuilder.append("-").append(UUID.randomUUID().toString());
        }
        System.out.println(stringBuilder.toString());
    }
}

反编译结果：

package com.noah.nowcoder;
import java.util.UUID;
public class Hello {
  public static void main(String[] args) {
    String str1 = "";
    for (int i = 0; i < 100000; i++)
      str1 = str1 + "-" + UUID.randomUUID().toString(); 
    System.out.println(str1);
    StringBuilder stringBu编程客栈ilder = new StringBuilder();
    for (int j = 0; j < 100000; j++)
      stringBuilder.append("-").append(UUID.randomUUID().toString()); 
    System.out.println(stringBuilder.toString());
  }
}

这一步结果不明显

字节码信息

接下来，我们看一下字节码信息

Compiled from "Hello.java"
public class com.noah.nowcoder.Hello {
  public com.noah.nowcoder.Hello();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return
  public static void main(java.lang.String[]);
    Code:
       0: ldc           #2                  // String
       2: astore_1
       3: iconst_0
       4: istore_2
       5: iload_2
       6: ldc           #3                  // int 100000
       8: if_icmpge     46
      11: new           #4                  // class java/lang/StringBuilder
      14: dup
      15: invokespecial #5                  // Method java/lang/StringBuilder."<init>":()V
      18: aload_1
      19: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lajsng/String;)Ljava/lang/StringBuilder;
      22: ldc           #7                  // String -
      24: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      27: invokestatic  #8                  // Method java/util/UUID.randomUUID:()Ljava/util/UUID;
      30: invokevirtual #9                  // Method java/util/UUID.toString:()Ljava/lang/String;
      33: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      36: invokevirtual #10                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      39: astore_1
      40: iinc          2, 1
      43: goto          5
      46: getstatic     #11                 // Field java/lang/System.out:Ljava/io/PrintStream;
      49: aload_1
      50: invokevirtual #12                 // Method java/io/PrintStream.printhttp://www.devze.comln:(Ljava/lang/String;)V
      53: new           #4                  // class java/lang/StringBuilder
      56: dup
      57: invokespecial #5                  // Method java/lang/StringBuilder."<init>":()V
      60: astore_2
      61: iconst_0
      62: istore_3
      63: iload_3
      64: ldc           #3                  // int 100000
      66: if_icmpge     91
      69: aload_2
      70: ldc           #7                  // String -
      72: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      75: invokestatic  #8                  // Method java/util/UUID.randomUUID:()Ljava/util/UUID;
      78: invokevirtual #9                  // Method java/util/UUID.toString:()Ljava/lang/String;
      81: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      84: pop
      85: iinc          3, 1
      88: goto          63
      91: getstatic     #11                 // Field java/lang/System.out:Ljava/io/PrintStream;
      94: aload_2
      95: invokevirtual #10                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      98: invokevirtual #12                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
     101: return
}

注意：

15: invokespecial #5 // Method java/lang/StringBuilder."<init>":()V

调用了 StringBuilder 构造函数，初始化了一个 StringBuilder 对象（循环中初始化对象）

总结

根据字节码改写代码

    public void add2() {
        String str1 = "";
        for (int i = 0; i < 100000; i++) {
            str1 += "-" + UUID.randomUUID().toString();
        }
        System.out.println(str1);
    }
    public void add3() {
        String str1 = "";
        for (int i = 0; i < 100000; i++) {
            StringBuilder stringBuilder = new StringBuilder();
            str1 = stringBuilder.append(str1).append("-").append(UUID.randomUUID().toString()).toString();
        }
        System.out.println(str1);
    }

字节码文件：

public void add3();
    Code:
       0: ldc           #10                 // String
       2: astore_1
       3: iconst_0
       4: istore_2
       5: iload_2
       6: ldc           #11    android             // int 100000
       8: if_icmpge     48
      11: new           #3                  // class java/lang/StringBuilder
      14: dup
      15: invokespecial #12                 // Meth编程客栈od java/lang/StringBuilder."<init>":()V
      18: astore_3
      19: aload_3
      20: aload_1
      21: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      24: ldc           #13                 // String -
      26: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      29: invokestatic  #14                 // Method java/util/UUID.randomUUID:()Ljava/util/UUID;
      32: invokevirtual #15                 // Method java/util/UUID.toString:()Ljava/lang/String;
      35: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      38: invokevirtual #9                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      41: astore_1
      42: iinc          2, 1
      45: goto          5
      48: getstatic     #16                 // Field java/lang/System.out:Ljava/io/PrintStream;
      51: aload_1
      52: invokevirtual #17                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      55: return

从字节码文件可以看出，add2() 和 add3() 基本上等价的。

所以循环中拼接字符串更高效的做法是将 StringBulider 放在循环外。

如下：

  public static void main(String[] args) {
    StringBuilder stringBuilder = new StringBuilder();
    for (int j = 0; j < 100000; j++)
      stringBuilder.append("-").append(UUID.randomUUID().toString()); 
    System.out.println(stringBuilder.toString());
  }

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